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Introduction to Linux - A Hands on Guide

This guide was created as an overview of the Linux Operating System, geared toward new users as an exploration tour and getting started guide, with exercises at the end of each chapter.
For more advanced trainees it can be a desktop reference, and a collection of the base knowledge needed to proceed with system and network administration. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. They hope these examples will help you to get a better understanding of the Linux system and that you feel encouraged to try out things on your own.

I just received an email from "A User"@{mydomain}.com with the subject "Re: {some subject}". There is no such user authorized (by me) to send email from my server. I'm also using spamassassin-2.44.11.8.x and it must not be properly configured because there is still a host of unwanted messages comming in.

I thought that I covered the bases as far as sendmail security was concerned. Is there a checklist that I can go through to make sure that I am less vulnerable? I'm running sendmail-8.12.8-9.90 on RedHat 9.0 kernel 2.4.20-20.9.

The "From" field on the email could have been forged. Take a look at your maillogs for any outgoing messages from that user. Also, the log entry for that particular incoming message should also indicate whether it was sent locally or remotely.